Various types of magnetic technology utilize magnetic elements for storing or reading data. For example, in conventional MRAM technology, the conventional magnetic element used is a spin tunneling junction. FIG. 1A depicts one such conventional magnetic element 10, which is a spin tunneling junction 10. The conventional spin tunneling junction 10 includes an antiferromagnetic (AFM) layer 12, a pinned layer 14, a barrier layer 16, and a free layer 18. The conventional pinned layer 14 is ferromagnetic and has a magnetization that is typically pinned by the AFM layer 12. As used herein, the term “ferromagnetic” includes ferromagnetic, ferrimagnetic, and sperimagnetic materials. The conventional ferromagnetic free layer 18 is separated from the pinned layer by the insulating barrier layer 16. The barrier layer 16 is sufficiently thin to allow tunneling of charge carriers between the pinned layer 14 and the free layer 18. Similarly, in conventional hard disk magnetic recording technology, the magnetic elements for magnetoresistive read heads include conventional magnetic elements, such as a spin valves. Spin valves have an analogous structure to the conventional spin tunneling junction 10. However, the barrier layer 16 is replaced by a conductive nonmagnetic spacer layer. Spin valves include a ferromagnetic pinned layer having a magnetization that is typically pinned by an AFM layer. The spin valve also includes a ferromagnetic free layer separated from the pinned layer by a conductive, nonmagnetic spacer layer, such as Cu. The ferromagnetic pinned and free layers of the spin tunneling junction and spin valve may also be synthetic.
In order to program the conventional spin tunneling junction 10, an external magnetic field is applied, typically by running a current through one or more write lines (not shown). In response to the magnetic field, the magnetization of the conventional free layer 18 aligns parallel or antiparallel to the magnetization of the conventional pinned layer 14. When the magnetic field is removed, the magnetization of the conventional free layer 18 remains in place. If the magnetization of the conventional free layer 18 is parallel to the magnetization of the conventional pinned layer 14, then the conventional spin tunneling junction 10 is in a low resistance state. If the magnetization of the conventional free layer 18 is antiparallel to the magnetization of the conventional pinned layer 14, then the conventional spin tunneling junction 10 is in a high resistance state. For example, suppose that when the magnetizations of the conventional free layer 18 and the conventional pinned layer 14 are parallel, the total resistance of the conventional magnetic element 10 is R−ΔR. Then, when the magnetizations of the conventional free layer 18 and the conventional pinned layer 14 are antiparallel, the resistance is R+ΔR. The conventional magnetic element 10 can thus be considered to have a median resistance of R. The median resistance is the resistance in the middle of the range of operation for the device. Based on these two states, R−ΔR and R+ΔR, one bit of information (corresponding to a zero or a one) is stored in the conventional spin tunneling junction 10.
In addition, other conventional magnetic elements build on the conventional spin tunneling junction 10. For example, a dual conventional spin tunneling junction could be used. In such a conventional magnetic element, a second pinned layer and a second barrier layer between the second pinned layer and the free layer 18 could be provided. Other conventional magnetic elements may use a conducting layer in lieu of a second barrier layer, as shown in FIG. 1B and FIG. 2. In such a case, the conventional magnetic element could be considered a combination of a spin valve and a spin tunneling junction which share a common free layer.
FIG. 2 depicts another conventional magnetic element 20 capable of storing multiple bits of data. The magnetic element 20 includes two spin tunneling junctions 30 and 40 separated by a conductive layer 22. The spin tunneling junction 30 includes a pinned layer 32 and a free layer 36 separated by a barrier layer 34. Similarly, the spin tunneling junction 40 includes a pinned layer 42 and a free layer 46 separated by a barrier layer 44. For clarity, AFM layers are not depicted. However, AFM layers are typically used to pin the magnetizations of the pinned layers 32 and 42. The conductive layer 22 electrically connects the spin tunneling junctions 30 and 40. The conventional free layers 36 and 46 are configured such that the magnetization of the conventional free layer 36 will change direction at a different magnetic field than the magnetization of the conventional free layer 46. This is typically accomplished by ensuring that the conventional free layers 36 and 46 have different thicknesses. In addition, the barrier layers 34 and 44 have different thicknesses so that the conventional spin tunneling junctions have different resistances.
In order to program the conventional magnetic element 20, external magnetic fields are applied, typically using current driven through one or more write lines (not shown). For the purposes of explanation, assume that the coercivity of the conventional free layer 36 is H1, while the coercivity of the conventional free layer 46 is H2. Also assume that H1 is less than H2. If a “00” is stored, a magnetic field, H, greater than H1 and H2 in a first direction, for example parallel to the direction of magnetization of the conventional pinned layer 32, is always applied first. Thus, the magnetizations of the conventional free layers 36 and 46 are parallel. In addition, the magnetizations of the conventional free layers 36 and 46 are both parallel to the magnetizations of the pinned layers 32 and 42. If a “10” is desired, then H is applied in the same direction and removed. A second field is then applied. The magnitude of the second field is between H1 and H2. The direction of the second field is opposite to the direction of H and, therefore, antiparallel to the magnetizations of the conventional pinned layers 32 and 42. Consequently, the magnetization of the conventional free layer 36 is antiparallel to the magnetization of the conventional pinned layer 32, while the magnetization of the conventional free layer 46 is parallel to the magnetization of the conventional pinned layer 42. If a “01” is desired, then H is first applied in the opposite direction, antiparallel to the magnetizations of the conventional pinned layers 32 and 42. The field is then removed. A field between H1 and H2 parallel to the magnetizations of the conventional pinned layers 32 and 43 is then provided. As a result, the magnetization of the conventional free layer 36 will be oriented parallel to the magnetization of the conventional pinned layer 32, while the magnetization of the conventional free layer 46 is oriented antiparallel to the magnetization of the conventional pinned layer 42. If a “11” is desired, then H is applied in the direction antiparallel to magnetizations of the conventional pinned layers 32 and 42. Thus, two bits corresponding to “00”, “01”, “10” and “11” are stored in the magnetic element 50.
The states “00”, “01”, “10”, and “11” correspond to different resistances. The resistance of the spin tunneling junction 30 is R1−ΔR1 when the magnetizations of the free layer 36 and pinned layer 32 are aligned or R1+ΔR1 when the magnetizations of the free layer 36 and pinned layer 32 are antiparallel. R1 can be considered to be the median resistance of the spin tunneling junction 30, and ΔR1 the change from the median resistance to one of the stable states (magnetizations parallel or antiparallel). The resistance of the spin tunneling junction 40 is R2−ΔR2 when the magnetizations of the free layer 46 and pinned layer 42 are aligned in parallel. The resistance of the spin tunneling junction 40 is R1+ΔR1 when the magnetizations of the free layer 46 and pinned layer 42 are antiparallel. R2 can be considered to be the median resistance of the spin tunneling junction 40, and ΔR2 the change from the median resistance to one of the stable states (magnetizations parallel or antiparallel). For the conventional magnetic element 20 to function as desired, R1 should be different from R2 and ΔR1 should be different from ΔR2. Thus, a “00” corresponds to the resistance R1−ΔR1+R2−ΔR2 for the conventional magnetic element 20. A “01” corresponds to the resistance R1−ΔR1+R2+ΔR2 for the conventional magnetic element 20. A “10” corresponds to the resistance to the resistance R1+ΔR1+R2−ΔR2 for the conventional magnetic element 20. A “11” corresponds to the resistance to the resistance R1+ΔR1+R2+ΔR2 for the conventional magnetic element 20.
Although a conventional magnetic memory using the conventional magnetic elements 10 and 20 using the conventional spin tunneling junction can function, one of ordinary skill in the art will readily recognize that there are barriers to the use of the conventional magnetic elements 10 and 20 at higher memory cell densities. In particular, the conventional magnetic elements 10, 20 are written using an external magnetic field generated by currents driven through bit lines (not shown) and the write lines (not shown). In other words, the magnetizations of the free layers 18, 36, and 46 are switched by the external magnetic field generated by current driven through the bit line and the write line. The magnetic field required to switch the magnetizations of the free layers 18, 36, and 46 known as the switching field, is inversely proportional to the width of the conventional magnetic elements 10 and 20. As a result, the switching field increases for conventional memories having smaller conventional magnetic elements 10 and 20. Because the switching field is higher, the current required to be driven through the bit line and particularly through the write line increases dramatically for higher magnetic memory cell density. This large current can cause a host of problems in the conventional magnetic memories using the conventional magnetic elements 10 or 20. For example, cross talk and power consumption would increase. In addition, the driving circuits required to drive the current that generates the switching field at the desired conventional memory element 10 or 20 would also increase in area and complexity. Furthermore, the conventional write currents have to be large enough to switch a magnetic memory cell but not so large that the neighboring cells are inadvertently switched. This upper limit on the write current amplitude can lead to reliability issues because the cells that are harder to switch than others (due to fabrication and material nonuniformity) will fail to write consistently.
Accordingly, what is needed is a system and method for providing a magnetic memory element which can be used in a memory array of high density, low power consumption, low cross talk, and high reliability, while providing sufficient read signal. The present invention addresses the need for such a magnetic memory element.